ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 377 pass made of quartz plates. For a reason already mentioned, sulphuret of carbon cannot be employed for filling the prisms', and the dispersive power of water is very low, but there appears to be no objection to the use of a solution of some colourless metallic salt. At least saturated solutions of sulphate of zinc and of acetate of lead, the only salts I have tried with this view, showed no defects of transparency when examined in quartz vessels by means of the flame of a spirit-lamp and a solution of sulphate of quinine*. Effect of Hydrochloric Acid, &c. on Solutions of Quinine. Optical evidences of combination in oilier instances. 205. Sir John Herschel, in his interesting paper already so often referred to, observes that it is only acid solutions of quinine which exhibit the peculiar blue colour, and that among different acids the muriatic seems least efficacious (page 145). For my own part I have tried solutions of quinine (not di-sulphate) in dilute sulphuric, phosphoric, nitric, acetic, citric, tarturie, oxalic, and hydrocyanic acids, and also in a solution of alum. In all these cases the blue colour of the dispersed light was plainly seen by ordinary daylight, especially when the fluid was examined by superficial projection. It was not easy to say which solution answered best, but I am inclined to think that in which phosphoric acid was used. 200. But when quinine was dissolved in dilute hydrochloric acid the blue colour was not exhibited, not even when the fluid wa,s held in the sunlight, and examined by superficial projection. Certain theoretical views led me to regard this as an evidence of a more intimate union between quinine and hydrochloric acid than between quinine arid the acids first mentioned, and to try whether the addition of hydrochloric acid to the solutions mentioned in the preceding paragraph would not destroy the blue colour. On trial this proved to be actually the case, so that even sulphuric acid is incapable of developing the blue colour in a solution of quinine in hydrochloric acid. * See note H.